This invention relates to a method of and apparatus for measuring the moisture content of a fluid system. More particularly, it relates to a moisture sensing element constructed of certain crystalline materials for use in such a method and apparatus.
The detection of the amount of water in a fluid stream is often of importance, e.g., in environmental and process control. Examples of areas where knowledge of the amount of water entrained in a stream are important include, but are not limited to, heating and air conditioning systems, dry gas streams, drying ovens and chemical reactions. Humidity sensors, to be effective, should have high sensitivity and selectivity, temperature insensitivity or compensation, good reversibility, fast response, highly stable response, infrequent need for calibration, and should tolerate corrosive and high temperature environments. The range of moisture levels which need to be detected may range from ppm or lower to 100% relative humidity. Moisture can be present in a fluid stream as water vapor, steam or present in a non-aqueous liquid. The term "fluid system" as used herein is intended to include air and other gases containing water and also liquids containing a small amount of water.
One useful category of instruments which may be employed to sense moisture in fluid systems are the electric hydrometers. As used herein, an electric hydrometer is an instrument for determining the moisture content of a fluid system by measuring or determining the electric impedance, or related property, of a hygroscopic material as an indication of the humidity or water content of the fluid system. By "electrical impedance" or "impedance" is meant that quantity as classically defined which takes into account at least one, and preferably both, of the electrical resistance and the electrical capacitance of a material or an electrical circuit. In instances where the electrical resistance of a material is very small, the electrical impedance measured is substantially the reciprocal electrical capacitance of the material. Conversely, where the capacitance of the material is very large, the electrical impedance measured is substantially the electrical resistance of the material. The term "electrical impedance" or "impedance" may include only electrical resistance or reciprocal electrical capacitance, provided that such value is a valid indicator of the humidity or water content of the fluid system being analyzed.
Zeolite molecular sieves are crystalline materials with a large and well defined interior volume. Access to this interior volume is controlled by openings, or pores, in the crystal. Molecules in the liquid or gas phase are adsorbed into the zeolite molecular sieves selectively on the basis of their size and polarity, among other things. Zeolite molecular sieves are aluminosilicates which contain charge balancing cations in the pore volume. These cations are mobile and thus render the zeolite molecular sieves ionic conductors.
Zeolite molecular sieves identified as zeolites X, A, Y, T, R, D, L, S, G, chabazite, erionite, faujasite, mordenite and analcite have been disclosed as humidity sensors in U.S. Pat. No. 3,186,225, which is incorporated in its entirety herein by reference. This patent discloses that the mobility of framework cations is a function of polar adsorbates and thus the resistance of a pressed powder compact of such zeolite molecular sieves equipped with electrodes can be monitored as a function of variable humidity levels. This patent also discloses that the lowest possible impedance gives the best results. The impedance of these systems decreases as the number and mobility of the charge balancing cations increases. In general, the zeolites disclosed in this patent have silica to alumina molar ratios of less than 6. Thus, the zeolite sodium X is preferred in this patent.